In recent years, performances of digital cameras and video cameras have increasingly been enhanced and, more specifically, their miniaturization is becoming greater and greater and their magnification and definition are becoming higher and higher. To achieve these enhanced performances, optical lens glasses used for digital cameras and video cameras have been increasingly required to have properties, such as high refractive index, high dispersion, and anomalous dispersion.
An example of glasses proposed which meets the above properties is a SiO2—Nb2O5-based glass having a refractive index (nd) of 1.78 or more and an Abbe's number (νd) of 30 or less (see, for example, Patent Literature 1). This glass can be used as a lens for optical communication by taking advantage of its properties, including high refractive index and high weatherability.
Known methods for producing an optical lens used for a digital camera, a video camera or the like includes a method of first molding a molten glass into an ingot, cutting the ingot into a glass material of appropriate size, polishing the glass material, and then press-molding it; and a method of forming a molten glass into a glass material by dropping a molten glass from a nozzle tip and forming it in a droplet shape, i.e., by so-called droplet forming, and then press-molding the formed glass material following or without polishing of the formed glass material.